The present invention relates to a method of bleaching pulp with peroxide. The invention especially relates to intensifying and at the same time simplifying the pretreatment required by peroxide bleaching.
Bleaching of pulp with peroxide is previously known in many connections. Especially in bleaching of chlorine-free pulp, peroxide has an important role. Prior to peroxide bleaching, it is necessary to remove heavy metals from the pulp by utilizing, for example, complexing agents such as EDTA or DTPA. It has been established in tests that a suitable pH value is 4 to 7, preferably 5 to 6, in this so-called chelating stage.
On the other hand, it has been surprisingly established (FI 944808) recently that the kappa number of pulp may be decreased by mere acid in a pH range of 2 to 6, preferably 3 to 4. The temperature has to be 60 to 130xc2x0 C. and the duration 20 to 240 minutes in this, so-called acidifying treatment. A suitable acid is aminic acid, sulphuric acid, or hydrochloric acid, even though other corresponding acids may be considered as well. In other words, we have found that, besides peracids suggested in Swedish patent 500605, which, as known, contain a delignifying perhydroxyl ion, a suitable acid may be some acid which does not contain any known delignifying ion or equivalent. A prerequisite for operation without peracids or equivalent is that the temperature is high enough (cf. FI 944808). Peracids and equivalent do not call for high temperatures; usually a temperature below 75xc2x0 C., most usually that of 50 to 75xc2x0 C., is sufficient. Treatment with acid may be intensified with additional chemicals, but it is once more to be noted that it is not at all necessary for decreasing the kappa number. Such additional chemicals are those which make the treatment of metals more effective or more effectively decrease the kappa number.
It has been often thought that acidification (A) and chelation (Q) stages could be united, but practical experience has shown that it is impossible. The pH ranges of acidification and chelation deviate from each other and, therefore, two separate treatment towers are necessary. At the acidification (A) stage, the pH has to be low enough, the temperature high enough and the treatment time long enough. At the chelation stage (Q), however, the pH has to be high enough. It is an object of this invention to provide a method of implementing acidification, decrease of the kappa number, and chelation, as simply and efficiently as possible, excluding unnecessary pumping operations.
It is previously known that prior to peroxide bleaching, the pulp is treated at a ZQ stage where, at the Z stage it is delignified with ozone at a pH of 2 to 4 and, thereafter, treated at the Q stage for removing metals. However, there is a problem of the Z stage being fast, usually taking less than 1 minute, and often cold, below 70xc2x0 C. The Z stage thereby provides poor conditions for dissolving metals from fibers. This may be partly remedied by adding an A stage prior to the ozone treatment so that an AZQ stage, i.e., an arrangement with three towers, is brought about. This arrangement involves two problems in view of dissolving metals. One is the temperature and the other concerns removal of heavy metals; the best way of removing heavy metals is to provide extracting time for pulp after delignification. In other words, with regard to metals removal, the ZQ stage should be an AZAQ stage, i.e., an arrangement with four towers, which should be run at a high temperature of preferably over 70xc2x0 C. But a further problem is involved in here, namely the ozone treatment produces radicals which are harmful to the pulp quality and which have enough time so as to react with pulp in the second A tower.
In Tappi Pulping Conference held in 1994, Nordgren and Elofson suggested in their paper xe2x80x9cNew process for metal ion chelation at elevated pH in pulp productionxe2x80x9d including an AQ stage for removal of metals. According to their teaching, the pH is 3 to 5 at the A stage and 6 to 9 at the Q stage. They suggest that the temperature of the process be 75xc2x0 C., which is too low in view of decreasing the kappa number. A weakness of the method by Nordgren and Elofson is thereby that after their AQ stage, the bleachability of the pulp is still rather poor since the kappa number has not been lowered at the A stage, which impairs dissolving of metals and weakens the bleachability.
In the 1994 Tappi Pulping Conference, the article xe2x80x9cMetal management in ECF bleaching and the effect of peroxide efficiency in the EPO stagexe2x80x9d studied the effect of chelating agents when these were added to the chlorine dioxide stage. It is established in the study that, chelating agents, when added to the chlorine dioxide stage, do not lessen delignification at the chlorine dioxide stage, but delay the increase of brightness instead. The EPO stage functions better if chelating agents are added to the chlorine dioxide stage preceding the EPO stage. So, it is suggested in the article that the DQEPO sequence may be improved by converting it into a DQEPO sequence, by adding one treatment stage, i.e., a Q stage. The research had been made using the temperature of 60xc2x0 C. at the D stage, which is too low in practice. The kappa number was also too high, i.e., nearly 30.
It is a characteristic feature of a preferred embodiment of the method of the present invention that, prior to the peroxide stage which is preferably pressurized, most preferably a pressurized two-tower peroxide stage, pulp is treated at a two-tower treatment stage where the kappa number of pulp is lowered in acidic, hot conditions, the pH being preferably 2 to 6 and the temperature 75 to 130xc2x0 C., and thereafter at a chelation stage, the pH being 4 to 9 preferably 5 to 6. Hereby, the bleachability of pulp is made optimal with regard to both metals and the kappa number prior to the peroxide stage.
It is a characteristic feature of another, alternative embodiment of the method of the invention that pulp, the kappa number of which has been lowered with a hot acid treatment, is treated with chlorine dioxide or some peracid at the same bleaching stage, but in a separate tower.
The characteristic features of the method of the invention appear from the accompanying claims.